1. Field of the Invention
The present invention relates to a liquid droplet jetting apparatus having a piezoelectric actuator, and the piezoelectric actuator.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2012-56288 discloses an ink-jet head which jets liquid droplets of ink as a liquid droplet jetting apparatus which jets liquid droplets from nozzles. The ink-jet head of Japanese Patent Application Laid-Open No. 2012-56288 includes a flow passage unit, and a piezoelectric actuator (an actuator unit) provided on the flow passage unit. In the flow passage unit, ink flow passages are formed to include a plurality of nozzles, and a plurality of pressure chambers in respective communication with the plurality of nozzles. The piezoelectric actuator is joined to the flow passage unit in such a manner as to cover the plurality of pressure chambers.
The piezoelectric actuator has a plurality of stacked piezoelectric layers, a plurality of individual electrodes formed on a surface of an uppermost piezoelectric layer, and a common electrode arranged between the uppermost piezoelectric layer and a lowermost or an intermediate piezoelectric layer. The plurality of individual electrodes are arranged on the surface of the uppermost piezoelectric layer in areas facing the plurality of pressure chambers, respectively. Further, the plurality of individual electrodes and the common electrode are formed of gold, respectively. The plurality of individual electrodes are connected to a wiring member (Flexible Printed Circuit: FPC) on which a driver IC is mounted, and the driver IC applies a drive voltage (a high potential) to the plurality of individual electrodes respectively. On the other hand, the common electrode is also connected to the FPC, but constantly kept at the ground potential. If the driver IC applies the drive voltage to a certain individual electrode to generate a potential difference between the individual electrode and the common electrode, then an electric filed acts on the piezoelectric layers in the portion sandwiched by the two electrodes to bring about a deformation. This deformation causes the corresponding pressure chamber to change in volume, thereby applying jetting energy (pressure) to the ink inside the pressure chamber.
However, when the potential difference occurs between the two electrodes sandwiching the insulating body, a phenomenon called migration may occur such that the metallic material of the electrodes is ionized, and the ionized metal is affected by the electric field to move within the insulating body. If the piezoelectric actuator is such as disclosed in Japanese Patent Application Laid-Open No. 2012-56288, then the material of the individual electrode, to which the high potential is applied, is ionized to move within the piezoelectric layers toward the common electrode at the ground potential. This migration should be restrained as much as possible because it can cause some fatal problems to the piezoelectric actuator such as decrease in insulation performance of the piezoelectric layers, short circuits between the electrodes, etc. In this regard, the individual electrodes and the common electrode are formed of gold, respectively, in the piezoelectric actuator of Japanese Patent Application Laid-Open No. 2012-56288. Gold is a very stable substance, and thus is one of the materials less likely to give rise to the migration.
However, gold is also a very expensive material, and gold electrodes are formed to be as thin as possible (for example, 1 μm in Japanese Patent Application Laid-Open No. 2012-56288). Further, increasing the thickness of the individual electrodes gives rise to such a problem as inhibiting the piezoelectric layers from deformation. In this regard, it is also preferable to form gold individual electrodes thinly. On the other hand, however, because piezoelectric layers made of ceramic materials have a low toughness, they are liable to crack. Then, when crack occurs in the piezoelectric layers, the thin individual electrodes are liable to break up. If the individual electrodes are completely divided due to the crack, then the potential is only applicable to a part of the individual electrode, thereby making it impossible to operate the piezoelectric actuator.